#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
int main(void)
{
    int px,py,qx,qy;
    double x=0,y=0;
    scanf("%d %d",&px,&py);
    scanf("%d %d",&qx,&qy);
    if(px>qx)
    {
        x=px-qx;
    }
    else
    {
        x=qx-px;
    }
    if(py>qy)
    {
        y=py-qy;
    }
    else
    {
        y=qy-py;
    }
    double ans=(x+y)/2;
    printf("%f",ans);
}